226 PAULINE H. DEDERER. 



apparently homogeneous protoplasm of the flat cells. They 

 were observed to flatten out and thus extend the membrane. It 

 seemed as though the cells were under unequal tension for as the 

 membrane grew wrinkles formed in cells that had formerly 

 been flat. The wrinkles could be differentiated from the cell 

 thickenings previously described, for they were obviously folds 

 in the membrane and often involved more than one cell. 



Ectodermal pseudopodia, formed from a hyaline outer region 

 of the cells, have been observed by Harrison ('10) and by Holmes 

 ('13) in tissue cultures of the frog. In the ectoderm of fundulus, 

 however, the entire cell is equally hyaline when stretched out 

 flat, and the formation of pseudopodia was never observed. 



The ectodermal membrane is not only very extensible, but 

 elastic as well. In staining the cultures with vital dyes the 

 greatest care was necessary to prevent its retraction, which 

 often followed within a few seconds. This is also likely to occur 

 if the slides are jarred. In one culture the ectoderm cells along 

 the edge contracted and thickened, pulling along with them 

 large portions of the mesenchyme membrane. This double 

 membrane then rolled in upon itself and, as it tore loose from the 

 cover-glass, could be seen adhering by short projections from 

 certain of the cells. These in turn loosened and the membrane 

 pulled in farther (Fig. 5). In such cases of mechanical disturb- 

 ance of the culture the membrane later became reduced by the 

 contraction of the ectoderm to a compact mass of cells which 

 eventually disintegrated. Ruth ('n) describes the contraction 

 of the edges of the growing epithelial cells during the healing of 

 a wound in the skin of a frog in vitro. 



Striations in Ectoderm Cells. The most peculiar characteristic 

 of the ectoderm cells is the presence of numerous delicate stria- 

 tions, more or less concentrically arranged, which form an 

 intricate pattern over all the cell (Figs. 3. and 10). The mark- 

 ings did not appear on all of the cells and only occasionally were 

 they sufficiently clear in the living cultures to be drawn with the 

 camera lucida. When the cells began to spread out and migrate 

 from the cut surface of the embryo, a few markings could be seen 

 in the flat clear portion of the cell (text-fig, i) and, as this region 



